The assembly of our 2.4L MIVEC engine is now complete. In our last installment, we focused on the cylinder head and induction system of our 4G64-based, E85-fueled engine. To recap the previous installment, we increased our cylinder head flow significantly with a custom port job, while installing Kelford 272-degree cams, Ferrea valve springs, an AMS F1i intake manifold, and a Mil-spec 65mm throttle body. With the engine reassembled, we dropped the new powerplant into our '06 Mitsubishi Lancer EVO IX and sought out the right turbo system to feed the beast.
While the engine was out, we knew an upgraded clutch would be necessary to handle the hefty increase in torque we were anticipating. Our solution was Exedy's Twin Disc HD clutch, arguably the most popular upgrade for high-horsepower, street-driven EVOs, delivering clutch pedal pressure that isn't rock-hard like many aftermarket single-plate units. Although the twin-disc clutch feels like the OEM clutch, it creates enough clamping force to handle over 535 lb-ft of torque. Cerametallic friction discs are utilized between its plates for superior resistance to slippage under harsh driving conditions. The entire clutch assembly mounts to the included lightweight chromoly flywheel, which helps to improve throttle response, while decreasing rotational mass by 15 pounds over our factory unit.
When selecting a turbocharger for our 2.4L engine, we began looking for one that complements the rest of our engine package, while providing solid power numbers and good boost response. Since we planned on driving our EVO regularly on the street, a laggy turbo was not on the agenda. Installing a modified EVO IX turbo was tempting, but we found that it wouldn't produce enough airflow at higher rpm to support our needs. Geoff Raicer, of Full-Race, strongly recommended the BorgWarner S300SX 83-75 twin-scroll turbo: a journal-bearing, oil-cooled turbo that retails for only $999-significantly less than comparable ball-bearing options. Geoff had recently tested this turbo on a near-stock 4G63T 2.0L engine with very promising results, and now offers a complete bolt-on kit for the EVO.
The BW S300SX turbo is advertised to support 400-800 hp-more than sufficient for our power goals. At first, we were surprised at the unusually large size of the turbo compared to similarly rated versions on the market, but the overall quality was very nice, with a backing plate and center section made of rigid cast iron and a shaft twice the size of those found in competitors' turbos-exactly what we expected from a large OEM supplier. The T4-flanged, twin-scroll exhaust turbine housing we opted for measures 1.00 A/R, which is capable of handling 24-29 psi of boost; housings measuring 0.91 A/R (for up to 24 psi), and 1.10 A/R (30 psi or more) are available options. It's important to note that using the BorgWarner turbocharger on our particular engine/manifold setup required notching of the motor mount bracket to provide clearance for the larger-than-factory compressor outlet.
When mated with a properly designed exhaust manifold, the turbine's twin-scroll design routes exhaust gases from cylinders 1 and 4 through a separate passage within the turbine housing than cylinders 2 and 3. Due to the firing order of the 4G63T (1-3-4-2), the exhaust pulses alternate to create a higher velocity and undisturbed exhaust flow into the turbine wheel from each divide, while avoiding interruption by "non-complementary" exhaust gas pulses. This beneficial effect delivers improved exhaust gas scavenging, throttle response, and ultimately more power throughout the rpm range in comparison to conventional turbos. After passing through the turbine wheel, exhaust gases exit through a three-inch Full-Race downpipe via a custom V-band flange that is welded to the housing by Full-Race. This not only allows a smooth transition to the downpipe, but also allows the turbo to be replaced with virtually any T4-flanged Garrett 40R-series turbo in a hurry. Although we don't plan on swapping turbos down the road, it's always nice to know 750 hp is only a few minutes away!
The compressor wheel of the S300SX 83-75 turbo features a 60mm inducer, 83mm exducer, and what BorgWarner refers to as its "Extended Tip Technology": extended tips on their compressor wheels that are claimed to provide improved airflow by making smaller compressor wheels perform like much larger wheels. It may sound like the usual marketing hype, but many in the drag racing community have confirmed gains in horsepower and torque.
Full-Race hand-fabricated the twin-scroll manifold used to feed our BorgWarner turbo from eight-gauge T304H stainless-steel, and then robotically TIG welded it to perfection. The runners are nearly equal length and attached to extra thick, stainless flanges to prevent cracking. The cylinder head flange features a nicely machined transition into the runners, ensuring smooth exhaust flow into the manifold. Two V-band flanges allow exhaust gas to pass through dual wastegates and out to the atmosphere through our custom-made dump tubes. Overall, the manifold is a beautiful piece and includes a lifetime warranty against cracking. To further improve exhaust gas velocity and reduce under-hood temperature, we added a ceramic exhaust coating to the manifold, turbine housing, and downpipe.
With our larger turbo in place, we were eager to turn up the boost and make more power, but before doing so, had to counteract the elevated charge temperatures resulting from increased pressure-a situation our factory intercooler wouldn't be able to handle. We placed a call to AMS Performance in Chicago, IL, and purchased their upgraded, direct-replacement, 20x12.4x3.5-inch, bar-and-plate intercooler kit. AMS has been selling their popular intercooler kit for years, but recently made a significant upgrade to the design. Whereas most aftermarket EVO intercoolers feature fabricated sheetmetal end tanks that do not provide the best transition into and out of cores, with the aid of SolidWorks software, AMS improved theirs with cast-aluminum end tanks, which offer minimal pressure drop and install with zero modification to the bumper beam. To complete the connection between our turbo, intercooler and throttle body, we used an AMS 2.5-inch intercooler pipe kit, including high-quality silicone hoses and stainless steel clamps. Some modification was required to work with our unique turbo/manifold/block/head combination, since the AMS kit is an OEM replacement for the EVO IX and its native components.
To control the pressure on both sides of our turbo system, we turned to Synapse Engineering for their wastegates and blow-off valve. Their products are unique in that they use a fast-acting piston actuator that is sealed by O-rings, in place of a conventional diaphragm. Twin 50mm wastegates attach via V-band clamps and vent excess exhaust from both sides of our twin-scroll manifold, while the Synapse blow-off valve mounts to our AMS intercooler pipe kit via a standard Tial 50mm flange. Both wastegates and the blow-off valve are plumbed using a "Boost Connect" kit from Synapse, designed to securely fasten hoses using quick-connect fittings that cannot easily be dislodged or shaken loose, once pushed into place.
Look for the fourth and final chapter of our 4G64 build, as we install an Aeromotive Stealth fuel system, Fragola fittings and steel-braided lines, and a Vi-Pec ECU tuned by Apex Speed Technology, and prepare to shake down the engine and put the BorgWarner twin-scroll to the test on the dyno.Will our MIVEC 4G64 have what it takes to produce a truly streetable 500 whp?